Since time immemorial, mankind has examined the heavens for signs of life. From the biblical wheel-in-the-sky of Ezekiel to Steven Spielberg's cuddly "E.T.," the possibility of extraterrestrial life has captured the popular imagination.
Scientists likewise have been intrigued. They have probed moon rocks and asteroids for bacterial fossils and spent decades sifting through signals from distant stars in hope of finding signs of alien civilizations.
Thus far, no real E.T.s.
Yet recent advances in astronomy, biology, and geology are spurring the most organized effort ever mounted to find extraterrestrial life.
Last week NASA announced a prestigious Nobel-Prize winner will head a new Astrobiology Institute. In the same week, researchers at the University of California at Berkeley unleashed what they call "the world's largest supercomputer," comprising more than 300,000 networked PCs, in a quest to locate radio signatures of intelligent life.
These are just two parts of an informal but vast network of prestigious scientists coalescing around the search for alien life. The network includes six other Nobel-Prize winners and scientists from almost every major research institution in the country.
At stake is not only an answer to the timeless question - "Are we alone?" - but also insights into how life evolves, what conditions support it, and whether mankind is unique in the universe.
"It's one of the greatest treasure hunts in history," says Lynn Harper, co-leader for astrobiology at NASA's Ames Research Center in Mountain View, Calif. "And why everyone is excited is not so much that the questions we're posing are new. It's that for the first time they may be answerable."
Scientists base their optimism on a confluence of advances that is bringing one of the great mysteries of life closer to resolution.
In 1995, astronomers located the first planets outside the solar system. Subsequent searches have turned up even more planets, increasing the likelihood that somewhere in the universe is one that could sustain life.
What's up on Mars?
Recent discoveries that Mars likely supported large bodies of water and may have had a system of sliding continental plates and an active internal magnetic geodynamo - all crucial to the development of life on Earth - indicate that life-supporting conditions existed at one time within our solar system.
Biologists have also learned life thrives under even the most trying circumstances, from bacteria buried deep within the Earth to spores frozen in polar ice to exotic tubeworms clustered around sulfur vents on the ocean floor.
"We are at the same stage as Galileo, Newton, and Leeuwenhoek were," says Dr. Baruch Blumberg, a Nobel laureate biologist and new head of the Astrobiology Institute. "Whenever they looked down the tube, they saw something new. Wherever we look, there is a new discovery."
Driving these advances is new technology. Astronomers now peer through telescopes equipped with advanced optical devices that allow them to blot out the bright light from stars in their search for planets. Astrophysicists can monitor hundreds of millions of radio channels simultaneously for sounds from alien civilizations.
Biologists manipulate tiny molecules and decode long strands of DNA far more rapidly and accurately than ever before - techniques that might allow them to identify and examine life on distant worlds.
More gee whiz
Miniaturization lets scientists place on spacecraft compact but complex devices that do everything from mapping the surface of planets in three dimensions to digging samples of rocks. Spacecraft themselves have become cheaper and easier to build.
"When I started my career, I never thought I would ever say we will have two missions to Mars every 26 months," says Ms. Harper. "You can do for a million dollars today what you could not do for any price a decade ago."
While recent progress has been impressive, it promises to accelerate in coming years as additional NASA and other projects come on line.
New space telescopes that will locate Earth-like planets will also allow scientists to better target their searches.
A new generation of inter-planetary spacecraft may allow man to drill deep into Mars to search for life. Or launch tiny submarines under bodies of water suspected to exist on Europa, a moon of Jupiter.
A plan to build a huge radio telescope that covers 2.5 acres will give scientists unparalleled ability to search for signals from intelligent life. And the new orbiting space station will allow scientists to test basic biological theories of the origins of life in a new environment.
"There is even the possibility of making miniature devices that collect DNA and can sequence it remotely," says Mr. Blumberg.
The new effort is already producing more idea sharing among researchers in disparate fields.
"We don't speak each other's languages and then we get forced together at meetings," says University of Arizona astronomer Nick Woolf. "That is wonderful. It guarantees that we start asking silly questions, which are often the best questions to ask."
When will we find ET?
To be sure, the first discovery of life may be years away. But even the search should produce new insights and knowledge.
And scientists are already addressing novel questions arising from recent discoveries, such as whether bacteria or other life could survive the extreme conditions of interstellar transport or whether the earliest life on Earth used oxygen from the beginning.
"I would go so far as to say that life elsewhere would be based on carbon chemistry," says senior researcher David Des Marais of NASA's Ames. "And we would need liquid water. Beyond that, once we get to molecules that are bigger, like proteins and DNA, there is a lot of room for variability."